Yes, you can make Roth contributions to a solo 401(k), and that’s an added benefit of saving for retirement in one. Most self-employed retirement plans, such as a Simplified Employee Pension (SEP), let you make only tax-deductible contributions to the account. In that case, the money grows tax-deferred and is then taxable when withdrawn in retirement.

With a solo 401(k), however, you may have a choice of making traditional tax-deductible contributions or after-tax Roth contributions (not all solo 401(k) administrators offer a Roth option). The Roth contributions don’t give you a tax break now, but you can withdraw the money tax-free in retirement. It works like a Roth IRA but without income limits that bar high-earners from contributing.

The contribution rules are a bit tricky. Self-employed people can contribute to a solo 401(k) as both the employee and the employer, and they are subject to two sets of contribution rules. As the employee, you can contribute up to the amount of your income for the year, with an $18,000 maximum (or $24,000 if you are age 50 or older). You can make either Roth or traditional contributions (or a combination) up to that limit.

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Because you’re the employer as well, you can also contribute up to 20% of your net self-employment income (your business income minus half of your self-employment tax) to the solo 401(k). Those contributions can only be traditional, tax-deductible contributions, not after-tax Roth contributions. The combined total for both types of contributions cannot exceed $53,000 in 2015 (or $59,000 if 50 or older). And you cannot contribute more than your self-employed income for the year.

Your solo 401(k) contributions may be limited if you also contribute to a 401(k) at another job—for example, if you have a full-time job at a company with a 401(k) in addition to freelance income. In that case, total employee deferrals to your 401(k) at your full-time job plus your solo 401(k) are limited to $18,000 for the year (or $24,000 if 50 or older), regardless of whether they are Roth or traditional contributions. But you can still contribute to the solo 401(k) as the employer, up to 20% of your net self-employment income, with a $53,000 maximum (or $59,000 if 50 or older). But again, those contributions must be traditional rather than Roth contributions.

Making some Roth contributions can be a good way to diversify your tax situation in retirement, especially if you earn too much to contribute to a Roth IRA and most of your retirement savings will be taxable when withdrawn. For more information about whether to make Roth or traditional contributions to a 401(k), see Invest in a Roth 401(k) If You Can. For more information about self-employed retirement-savings options, see Retirement Plans for Self-Employed People.